More than a third of adults ages 55 and older report sleeping less than 7h per night on a regular basis, with nearly 15% sleeping <6h per night. The consequences of acute sleep deprivation are well understood, but far less is known about the effects chronic insufficient sleep, particularly in older people. Evidence gathered during the previous grant periods strongly suggests that the sleep quality of older people is extremely vulnerable to a misalignment between the timing of sleep and the phase of the circadian timing system suggests that older people will be less able than young adults to obtain needed recovery sleep following acute sleep loss. There is also compelling evidence that the interaction between the two major sleep regulatory processes, the circadian and the sleep-wake homeostat, changes with age. We propose a study that will assess the effects of chronic sleep restriction in young and older adults, using a study design that will allow us to quantify age-related differences in the impact of circadian and sleep-dependent influences on sleep. The proposed investigation is designed to test the following hypotheses: 1) that notwithstanding the increased homeostatic sleep pressure induced by chronic sleep restriction, sleep will be significantly more disrupted when it is scheduled at adverse circadian phases in older as compared with young adults;2) that under conditions of chronic sleep restriction, slow-wave activity during sleep will be increased at all circadian phases in both young and older subjects;3) that the circadian rhythm of REM sleep will be preserved across 4 weeks of chronic sleep restriction in young and older subjects, and that it will be preserved in older subjects at the expense of other sleep stages, including slow-wave sleep;and 4) that unlike the sleep of young subjects, during sleep extension following 4 weeks of chronic sleep restriction, the sleep of older subjects will not show rapid recovery as measured by the duration and intensity of slow-wave sleep, sleep efficiency, and number of awakenings. We propose to test these hypotheses in healthy young and older men and women whose sleep-wake cycle is scheduled to a 28-h day under conditions of chronic sleep restriction (i.e., 22.17 h wake followed by 5.83 h sleep opportunity) on a forced desynchrony protocol across 4 weeks. Polysomnographic sleep recording and quantitative analysis of EEC will be used to describe age-related changes in circadian sleep regulation and to understand the effects of chronic sleep restriction on sleep in older persons. The results of this project will provide important information on the effects of chronic insufficient sleep in older people, a condition which a significant portion of the older population experiences on a regular basis, and an area of investigation in which few laboratory-based investigations have been conducted.